Sectional drive



March 8, 1938. P. SIMONDS SECTiONAL DRIVE Filed July 6, 1936 2v In INVENl'EIR PAUL SIMDNDS A'r-rnnw v.

r hm Patented Mar: 8, 1938 PATENT OFFICE SEO'I'IONAL DRIVE Paul Simonds, Milwaukee, Wis, assignor to The Oilgear Company, Milwank -tion of Wisconsin cc, Wia, a corpora Aimlication July s, 1936. Serial No.89...

11 Claims. (01. 60-53) This invention relates to drives of the'type employed to drive a group of related machines or the several units or sections of a single machine, such as a paper machine, printingpress or the like. The torque required to start such a machine or group of machines is often as much as twice the running torque so it is ordinarily necessary to drive the same from a power plant having a capacity considerably in excess of the power required to keep the machine or group of machines running after being started.

The present invention has as an object to provide a sectional drive which will maintain the speeds of the several machines or machine units driven thereby proportional to each other.

Another object is to provide a drive in which a plurality of hydraulic motors may be driven by liquid supplied thereto from a single source and the speeds of the several motors maintained proportional to each other.

Another object is to provide a drive which may be adjusted to vary the speed of one or more of its sections relative to the speed of one or more other sections in order to provide draw between adjacent machines or machine units driven thereby.

Another object is to provide a sectional drive which will enable a. prime mover to drive a group of machines vor machine units at proportional 30 speeds and which may be adjusted to vary the overall speed of the group without changing the ratio between the speeds of the individual machines or machine units and without varying the speed at which the drive is driven by the prime mover. W

Another object is to provide a sectional drive which will permit a group of machines or machine units to be started and driven by a power plant of considerably smaller capacity than the power plant heretofore required to start and drive a similar group of machines or machine units.

Other objects and advantages will appear from the description hereinafter given of a hydraulic drive in which the invention is embodied.

According to the invention in its general aspect, the drive is divided into several sections each of which includes a variable displacement hydraulic motor and a hydraulic helper motor which 50 are mechanically synchronized, all the helper motors are hydraulically connected in series and supplied with motive liquid from a single source, the variable displacement motorsfare supplied with motive liquid from a difierent source, and

55 each variable displacement motor has its displacement varied in response to variations in the drop in pressure across its helper motor.

The invention is exemplified by the drive shown schematically in the accompanying drawingin which the views are as follows: 1 5

Fig. 1 is a diagram of, the hydraulic circuit I and shows the relation between the several parts of the drive.

Fig. 2 is a section through one of the motors, the view being somewhat diagrammatic in char- 1o acter and showing the displacement varying mechanism considerably enlarged in respect to the motor.

The drive may be provided with any number of sections but, for the purpose of illustration, it 5 has been shown-provided with three sections A,

B, and C to drive, respectively, three related machines or three units I, 2, and 3 of a single machine. I

Each drive section includes a variable displace- 2o ment hydraulic motor 4 and a second hydraulic motor 5 which are mechanically connected to each other and to the machine to'be driven. Motor 4 has been designated as the "main motor and shown as being considerably larger than mo- 25 tor 5 which has been designated as a "helper" motor but such proportions are not necessary for the reason that the two motors may be of the same or of difierent sizes as long as neither motor is capable oi driving both the machine and the other motor.

The motors of each section may be connected in any suitable manner to each other and to the machine to be driven by that section, as by means of a drive 6 which connects motor 4 to the machine and a drive I which connects motor 5 to motor ,4. Liquid for driving all of motors l is supplied thereto from a common source such as a pump III of the constant pressure type. Such a pump has 40 the characteristic of delivering liquid at its full volumetric rate until pump pressure reaches a predetermined maximum and then automatically reducing itsv displacement until it is delivering just sumcient liquid to maintain that maximum pressure substantially constant. Since pumps 0! this character are well known and in extensive use, no further description thereof will be given.

Pump Ill delivers its output through a channel -i I into a supply channel I! having three branches each of which is connected to the intake port of amotor 4. Liquid is returned to pump ill from motors I through a return channel l3 which connects the intake of pump III to a return channel I having three branches each of which is con- 4:

nected to the outlet of a motor 4.- All of the I lei with each other.

Liquid for operating motors 5 is supplied thereto by a variable displacement pump H which has its outlet connected to the inlet of motor of section A by a channel It and its inlet connected to the outlet of motor 5 of section C by a channel i9. The outlet, of motor 5 of section A is connected by a channel 23 to the inlet of motor 5 of section B which has its outlet connected by a channel 2| to theinlet of motor 5 of section C.

All of motors 5 and pump I! are thus connected in series with each other and each is preferably provided with means for varying the displacement thereof.'such as the stroke changing mechanism shown in Patent No. 1,998,984 and indicated in Fig. l by a hand wheel 22.

- Pumps II and H are shown as being arranged upon a base 24 and driven in unison by an electric motor 2! arranged thereon. Base 24 may be hollow and contain a supply of liquid for pumps i and I1.

Liquid for supercharging pumps l0 and I1 and for control purposes is supplied by an auxiliary pump such as a gear pump 23 which may be incorporated in one of the pumps and driven in the return sides of the circuits of pumps in and IL A single resistance valve may be employed for this purpose but, in order to keep the two circuits independent of each other, the drive is shown provided with a resistance valve 39, which has its inlet connected to channel l4 intermediate the ends thereof and its outlet connected to one end of a channel 3i, and a resistance valve 32 which has its inlet connected to the'other end of channel 3| audits outlet connected to 'a drain channel 33 which discharges into base 24. Channel 3| is connected intermediate the ends thereof to channel l9 intermediate the'ends thereof by a channel 34. Gear pump 23 is thus enabled to maintain in channel l4 a pressure equal to the combined resistances of valves 33 and 32 and to maintain in channel I! a pressure equal to the resistance valve 32. r v Themotprsl 4 may be of any suitable .type such as the rolling piston type shown in Patent No. 1,998,984. A hydrodynamic machine of this character has its-pistons and cylinders arranged in a rotatable cylinder barrel 40 (Fig. 2) which is journaled upon a stationary valve shaft or pintle 4| through which liquid flowsto and from the cylinders. The outer ends of the pistons engage an annularreaction surface 42 formed upon the inside of a slide block 43 in which cylinder barrel 40 is arranged.

when reactionsurface 42 and cylinder barrel 49 are concentric with each other, motor displacement is zero and motor 4 will remain idle. When slide block 43 is shifted so that reaction surface 42 is eccentric to cylinder barrel 40, motor displacement will be proportional to the 'distance slide block 43 is shifted from its zero displacement position and motor 4 will operate when supplied with motive liquid. Since pumps and motors of this type are well known and in extensive commercial use, no further description thereof will begiven.

' Slide block 43' is urged toward azero displacement position by a substantially constant force such as a spring or, as shown, by means of a piston -44 connected to slide block 43 and fitted in a stationary cylinder 43 which is ordinarily carried by the pump casing and -to which liquid is supplied at a constant prwsure by gear pump 23. To this end, each cylinder 45 is connected to a branch of channel i4 by a channel 43 so that 'a pressure equal to the combined resistances of valves 30 Land 32 prevails in each cylinder 4! whenever the drive is in operation. a

Slide block 43 of the motor ,4 in each drive section is adapted to be moved toward maximum displacement position in response to variations in the drop in pressure across the motor of the same section. For the purpose of illustration, each motor 4 is shown provided with an actuator II which has its casing connected to or formed integral withthe casing of. motor 4 and which is shown in detail in Fig. 2.

Actuator so is provided with a piston n which is connected to slide block'43 and adapted to be operated by liquid under the control of.a valve 52 which is shown on a considerably enlarged scale in respect to the motor. Pistons 44 and ii are also shown on an enlarged scale in respect to the motor.

Piston ii is fitted in a cylinder l3 which is formed-in the actuator casing and connected by a port 54 to a bore 55 which is formed in the actuator casing at right angles to cylinder 53 and has valve 52 arranged therein.

Valve 52 is provided with a centralhead or piston 56, which normally covers port 34, and two end heads or pistons 51 and 53 which are spaced from valve piston 53. All three of the valve pistons are closely fitted in bore I to reciprocate therein.

from gear pump 26 by means of a channel ll having one of its ends connected to channel l4 and its other end connected to the actuator casing in communication with bore 5! at a point which remains between valve pistons 33 and 31 in all positions of valve 32. Valve piston controls communication between port 84 and channel I3 and also between port 54 and a drain channel 33 communicating with bore 33 at a point which always remains between pistons 59 and 33 in all positions of valve I2.

In order that valve 32 may be operated in response to variations in the drop in pressu e across motor I, the upper end of bore II is connected to the intake side of motor 5, the lower end of bore 55 is connected to the discharge side of motor I, and valve 52 is urged upward by a spring 33 which is arranged in the lower end of bore 3! and has is tension adjusted by a screw 34. As shown, actuator 59 of section A has the upper end of its bore SI connected to channel II by a channel and the lower end thereof connected to channel 29 by a channel 33, actuator 33 of section B has the upper end of its bore 35. connected to.channel 29 by a channel 31 and the lower end thereof the pressure drop across motor 5 is normal.

The arrangement is such that the pressure prevailing at the intake of motor 5 and acting on valve piston 51 urges valve 52 downward against the upward forces exerted upon valve piston 58 by spring 63 and by the pressure prevailing at the outlet of motor 5. Spring 63 has its tension so adjusted that it will support valve 52 in such a position that valve piston 56 covers ports 54 when Tha is, with valve piston 56 in position to cover port 54, the force exerted by spring 53 is just equal to the difference between the forces exerted by the liquid upon valve pistons 51 and 58 when motor 5 uid is delivered thereto at a constant volumetric rate. x

If the load should increase, the pressure at the intake of motor 5 would increase and cause valve 52 to be depressed until port 54 was opened to channel 59. Then liquid from gear pump 26 would enter cylinder 53 and move slide block 43 toward the right and thereby increase motor displacememt) until motor 4 could carry enough of v the addit nal load to reduce the load on motor 5 until the drop in pressure across motor 5 was reduced to normal. Then the forces exerted upon valve piston; 58 by the liquid and by spring 63 would raise valve 52 until valve piston 56 covered port 54 and trapped the liquid in cylinder 53 to hold piston 5| and slide block'43 positions.

If the load should decrease, the pressure at the intake of motor 5 would ,decrease and permit the forces exerted upon valve piston 58 by the liquid and by spring 63 to raisevaive 52 until port 54 was opened to drain channel 60. Then liquid could escape from cylinder 53 and permit liquid from gear pump 26 to enter cylinder and move piston 44 and slide block 43 toward the left and thereby decrease motor displacement until motor 4 would be carrying a smaller share of the load and causing the load on motor 5 to be increased in their new until the drop inpressure across mptor 5 would increase to normal. Then the increased pressure at the inlet of motor 5 would cause valve 52 to move downward until valve piston 56 covered port 54 and trapped the liquid in' cylinder 53 to hold piston 5| and'slide block 43 in their new positions.

Since it is sometimes desirable to operate one or more sections of the drive while one or more three sections, and that valves I3, I4, and I5 are closed, the drive will operate as follows:

When electric motor 25 is energized, it will drive pumps I0, I1, and 26 which will deliver liquid into channels II, II, and 29 respectively. Pump III will attempt to drive all of the motors 4 but cannot do so as they do not have suilicient power to drive machine units I, 2, and 3 without the aid of motors 5. Also, some of the motors 4 will have reduced torque due to those motors having been cient liquid to maintain that maximum pressure substantially constant.

When motor 25 was previously deenergized to stop the apparatus, the inertia of the machine units I, 2, and 3 drove or tended to. drive the motors 4 and 5 and reversed the pressure in each circuit, thereby permitting springs .63 to raise valves 52 to open ports 54 to drain channels 60 so that when gear pump 26 was started, liquid discharged therefrom could enter the cylinder 45 in each of sections B and C and reduce the displacement of the motors 4 of those sections to minimum displacement. r

Gear pump liquid would also tend to reduce the displacement of motor 4 in section A but, since the pumps are all started at the same time, pump II will deliver liquid to motor 5 of section A which will attempt to start machine unit I but cannot do so at first due to the static friction and inertia.

thereof. Consequently, pump I'I will raise a high pressure in channel I8 and this pressure will extend through channel 65 and move valve 52 downward to open port 54 to channel 59 so that gear pump liquid willenter cylinder 53 and move slide block 43 toward maximum displacement position, thereby causing motor 4 to exert a high torque which, together with the torque exerted by motor 5, is ample to start machine unit I.

As soon as section A starts to operate, the liquid discharged from the. motor 5 thereof will flow through channel 20 to motor 5 of section B and attempt to start it but cannot do so due to the static friction and inertia of machine unit 2, Consequently, the pressure in channel 20 will rise and liquid will flow through channel 61 to actua-i tor -of section B and move its valve 52 downward, thereby permitting gear pump liquid to move slide block 43 toward maximum displace-- ment position until motor 4 of section B is exerting a high torquewhich, together with the torque exerted by motor 5, is ample to start machine unit 2. I

As soon as section B starts to operate, the liquid d scharged from the motor 5 thereof will flow through channel 2| and cause section C to start machine'unit 3 in the same manner that section B started machine unit 2. If the drive had more than three sections, the additional sections would be successively started in the same manner;

The displacement of pump I1 is preferably reduced, as by turning the hand wheel 22 thereof,

before motor 25 is energizedso that pump I! when started will at first deliver liquid to motors 5 at a. reduced rate and thereby cause the drive to operate at slow speed. After the drive has been started, it may be accelerated to the desired speed .by'increasing the displacement of pump IT.

Since the several machine units are started successivelyand at slow speed, it is not necessary that electric motor 25 and pumps l0 and I1 have the power capacity that would be required if the units were started simultaneously as in the prior drives.

-As-soon as a unit is started, less torque is required and, as soon as it has been accelerated to its running speed, still less torque is required.

Since motor 4 is supplied with liquid at a sub- -stantially constant pressure, it exerts a substantially constant torque as long as its displacement remains unchanged. Consequently, any decrease in the requiredtorque is first ielt'by motor I with 'a resultant drop in pressure at its inlet. This drop in pressure at the inlet of motor I causes actuator II to operate and eiiect a decrease in the displacement of motor 4 in the previously described manner until each of motors 4 and I is carrying its predetermined share of the load as determined by the adjustment of spring 63.

I. the ratios of drives 6 and I and the displacement of motor I in one section are exactly the same, respectively, as theratlos of drives 6 and I and the displacement of motor I in each of the other sections, all of the'machine units will be driven at the same speed for the reason that motors I are connected in series and must go operate at the same speed, any variation in load which might cause a variation in the speed of any unit being compensated for by adjustment of motor 4 as previously explained.

If it is desired toincrease or to decrease 'the 25 overall speed of the drive, the displacement oi pump i1 may be increased or decreased. If it is desired to vary the speed of one unit relative to the speed of. another unit for the purpose, for example, of compensating for the stretching or shrinking of the material being processed by the several units, it is simply necessaryto vary the displacement of motor I oi'that section as by adjusting hand'wheel 22. It the displacement oi motor I oi! one section is decreased, the speed oi the unit driven by that section .will be increased and, conversely, if its displacement is increased thespeed oi the unit will be decreased for the reason that pump 11 delivers liquid to motor I at a constant volumetric rate.

When the drive is stopped, .as by deenergizing electric motor II, the machine units driven by the'drive may have sufllcient inertia to drive motors 4 and I, thereby causing motors 4 and] to function as pumps and-tend to drive pumps II and I1 and electric motor II the resistance of which would cause a reversal of pressure in the circuit.

lithe machine should drive motors 4 and I and no preventative means were provided, motors 4 and I would discharge through resistance .valves ll and I! and thereby cause a deilciency oi liquidinthecircuitunlesstheresistancevalveswere adjusted to open at a pressure higher than the pressure requiredto drive pumps II and ii.

In order toprevent such a deficiency of liquid and to avoid the necessity of maintaining a relatively high prusure in the return side 01' each circuit, there may be provided between channels II and it a resistance valve II which is adjusted to open at a pressure lower than the pressure re-- quired to open resistance valve 82, and between channels it and i4 a resistance valve 11 which isadjustedtoopenatapressurelowerthanthepressure required to overcome the combined resistances of valves SI and 32. Valves II and II .5 would permit liquid to flow from channel ll 'to channel II and from-channel [4 into channel I! but would prevent it from flowing in the opposite.

direction.

Then when motors 4 and I were driven by the machine units, the liquid discharged by motor I oi seetion C would flow through channel ll, resistance valve Wand channel II to motor I of section As Resistance valve II would function would cause pressure to be created in channels is, II, and andthis pressure would extend into the lower ends of actuators I0 and raise the valves I2 thereof to open drain pipe 60.

The liquid discharged by motors 4 would flow from channel l4 into channel I! through resistance valve II which would cause pressure to be maintained in channel l4 and this pressure would extend through channels 48 into cylinders 4I and cause pistons 44 to move slide blocks 43 toward the left until motors 4' were at zero displacement or, if desired, to a predetermined minimum diseach cylinder II to placement in which case resistance valve I1 would also function as a brake to decelerate the machine units.

The invention herein set forth is susceptible of various modifications and adaptations without departing from the scope thereof as here- 7 after claimed.

The invention is hereby claimed as follows:

1. A sectional drive, comprising a series of drive sections each including a variabledisplacement hydraulic main motor and a hydraulic helper motor mechanically connected to each other, means for supplying motive'liquid to said main motors'at a pressure not exceeding a predetermined maximum, ,a pump for supplying motive liquid to said helper motors, fluid channels connecting said helper motors to said pump in series withpeach other, and means responsive to variations in the drop in pressure across each helper motor for varying the displacement of the main motor connected thereto.

2. A sectional drive, comprising a series of drive sections each including a variable displacement hydraulic main motor and a hydraulic helper motor mechanically connected to each other, means for supplying motive liquid to said mainmotorsatapressurenotexceedingapre-,

determined maximum, a pump for supplying motive liquid to said helper motors, fluid channels connecting. said helper motors to said pump in series with each other, means responsive to variations in the drop-.in pressure'across each helper motor for varying the displacement oi the main motor connected thereto, and means for varying the displacement 01' at least one helper motor to thereby vary the speed 0! the drive' section containing that motor relative to the speeds 01' other drive sections.

3. A sectional drive, comprising a series of drive sections each including a variable displacement hydraulic main motor and a hydraulic helper motor mechanically connected to each other, means tor supplying motive liquid to said main motors at a pressure not exceeding a predetermined maximum, a pump for supplying motive liquid to said helper motors fluid channels connecting said helper motors to said pump in series with each other, means responsive to vari-- the displacement oi said pump to thereby vary the speeds or all of said drive sections.

- 4. A. sectional drive, comprising a series oi drive sections each including a variable displacement hydraulic, main motor and a hydraulic helper motor mechanically connected to each.

other, means for sumilying motive liquid to said main motors at a pressure not exceeding a predetermined maximum, a pump for supplying motive liquid to said helper motors, fluid channels connecting said helper motors to said pump Iahahtodaceleratetbamachineunitsand inseries witheachothenmeansresponsivoto variations in the drop in pressure across each helper motor Iorvarying the displacement of the main motor connected thereto, means for varying the displacement of at least one helper motor unit and including a variable displacement hydraulic main motor and a hydraulic helper motor mechanically connected to each other and to said unit, means for supplying motive liquid to said main motors at a pressure not exceeding a predetermined maximum, eachooi said main motors when supplied with motive liquid at or below said maximum pressure having insuflicient power to drive the unit to which it is connected, a pump "-for supplying motive liquid to said helper motors,

fluid channels connecting said helper motors to said pump in series with each other, and means responsive to variations in the drop in pressure 6. A sectional driveior a series 01'. machine units, comprising a drive section for driving each unit and including a variable displacement hydraulic main motor and a hydraulic helper motor mechanically connected to each other and to said unit, means for supplying motive liquid to said main motors at a pressure not exceeding a predetermined maximum, each of said main motors when supplied with motive liquid at or below said maximum pressure having insuiflcient power to drive the unit to which it is connected, 9. pump for supplying motive liquid to said helper motors, fluid channels connecting said helper motors tosaid pump in series with. each other,

means responsive to variations in the drop in pressure across each helper motor for varying the 'displacement of the main motor connected thereto, and means for varying the displacement of at least one helper motor to thereby vary the speed of the drive section containing that motor relative to the speeds of other drive sections.

7. A sectional drive for a series of machine units, comprising a drive section for driving each unit and including a variable displacement hydraulic main motor and a hydraulic helper motor mechanically connected to each other and to said unit, means for supplying motive liquid to said main motors at aipressure not exceeding a predetermined maximum, each 'of said main motors when/supplied with motive liquid at or below said maximum pressure having insufflcient power to drive the unit to which it is connected, a pump for supplying motive liquid to said helper motors, fluid channels connecting said helper motors to said pump in series with each other, means responsive to variations in the drop in pressure across each helper motor for varying the dis- .p'lacement oi the main motor connected thereto,

and means for varying the displacement or said pump to thereby vary the speeds of alloi. said drive sections.

8. A sectional drive /for a series of machine units, comprising a drive section for driving each unit and including a variable displacement byi draulic main motor and a hydraulic helper motor mechanically connected to each other and to said-unit, means for supplying motive liquid to said main'motors at a pressure not exceedingaa predetermined maximum, each of said main motors when supplied with motive liquid at or below said maximum pressure having insumcient power to drive the unit to which it is connected,

a pump for supplying motive liquid to said helper motors, fluid channels connecting said helper motors to said pump in series with each other, means responsive to variations in the drop in pressure across each helper motor for varying the displacement of the main motor connected thereto, means for varying the displacement of at least one helper motor to thereby vary thespeed of the drive section containing that motor nels connecting all of said main motors to said pump in parallel with each other. a second pump, fluid channels connecting said helper motors to said secondpump in series with each other, and means responsive to variations in the drop in pressure across each helper motor for varying the displacement of the main motor connected thereto.

10. A sectional drive for a series of machine units, comprising a drive section for driving each unit and including a variable displacement hydraulic main motor and a hydraulic helper motor mechanically connected to each other and to said unit, a constant pressure pump, fluid channels connecting all of said main motors to said pump in parallel with each other, a second pump.

fluidchanneis connecting said helper motors to said second pump in series with each other, means responsive to variations in the drop in pressure across each helper motor .for varying the displacement of the main motor connected thereto, and means for varying the displacement of each helper motor to thereby vary the speeds of said drive sections relative to each other.

11. A sectional drive for a serieslot machine units, comprising a drive section for driving each unit and including a variable displacement hydraulic main motor and a hydraulic helper motor,

mechanically connected to each other and to said unit, a constant pressure pump, fluid channels connecting all ofsaid main motors to said pump in parallel with each other, a second pump, fluid channels connecting said helper motors to said second. pump in series with each other,

means responsive to variations in the drop in pressure across each helper motor for varying the displacement of the main motor connected thereto, and means for varying the displacement of said second pump to thereby vary the speeds of all of said drive sections.

12. A sectional drive for a series of machine units, comprising a drivesection-Ior driving each a unit and including a variable displacement bydraulic main motor and a hydraulic helper mo tor mechanically connected to each other and to said unit, a constant pressure pump, fluid channels connecting all of said main motors to thereto, means for varying the displacement 01' each helper motor to thereby vary the speeds oi! said drive sections relative to each other, and means for varying the displacement of said secondpumpto therebyvarythespeedsoiall or said drive sections.

13. In a hydraulic drive, the combination of a main hydraulic motor having an element shittable to vary motor displacement, means for exerting a substantially constant force upon said element to urge it toward zero displacement position. displacement varying means for moving said element in the opposite direction against said force, a helper motor y connected to said main motor to assist it in driving a load, means for supplying motive liquid to each of said motors to drive the same, and means responsive to variations in the drop in pressure across said helper motor for operating said displacement varying means.

14. In a hydraulic drive, the combination of a main hydraulic motor having an element shiftable to vary motor displacement, means for exerting a substantially constant force upon said element to urge it toward zero displacement posltion, displacement varying means for moving said element in the opposite directionagainst said force, a helper motor mechanically connected to said main motor to assist it in drivin a load, a constant pressure pump for supplying liquid to said main motor to drive the same, a volumetric pump for supplyingliquid to said helper motor at a measured-rate, and means responsive to variations in the drop in pressure across said helper motor for operating said displacement varying means.

15. In a hydraulic drive, the combination of a main hydraulic motor having an element shittable to vary motor displacement, means for exerting a substantially constant force upon said element to urge it toward sero displacement position, displacement varying means for moving said element in the opposite direction against said force, a helper motor mechanically connectrdtosaidmainmotortoasdstitindriving a load, a constant pressure pump for supplying liquidtosaidmainmotortodrivethesamaa volumetric pump for supplying liq d to said helper motor at a measured rate, means responsive to variations in the drop in pressure across said helper motor for operating said displacement varying means, and means for varying the displacement of said volumetric pump to thereby vary the speeds of said motors.

16. Ina hydraulic drive, the combination of a main hydraulic motor having an element shiftable to vary motor displacement means for exerting a substantially constant force upon said 10 element to urge it toward zero displacement position, displacement varying means for moving said element in the opposite direction against said force, a helper motor mechanically connected to said main motor to assist it in'driving a lo load, a constant pressure pump for supplying liquid to said main motor to drive the same, a volumetric pump for supplying liquid to said helper motor at a measured rate, means re sponsive to variations in the drop in pressure across said helper motor for operating said displacement varying means, and means for varying the displacement of said helper motor to thereby vary its speed and the speed of said main motor.

1'1. In'a hydraulic drive, the combination oi! a main hydraulic motor having an element shittable to vary motor displacement, means for exerting a substantially constant force upon said element to urge it toward zero displacement position, displacement varying means for moving said element in the opposite direction against said force, a helper motor mechanically connected to K said main motor to assist it in driving a load, a

constant pressure pump for supplying liquid to 85 said main motor to drive the same, a volumetric pump for supplying liquid to said helper motor atameasured rate,meansresponsivetovarlations in the drop in pressure across said helper motor for operating said displacement varying 40 

